3 Answers

Only the people who wrote that section of the TERPS will know definitively, but the easy answer is that 250 feet is the lowest they could get while maintaining required obstacle clearances given the resolution of the data given to the pilot (the needles) and the potential errors in that data.

In other words – They decided they couldn’t do 200 feet safely, but they could do 250 feet safely – So why would they round up to 300 feet absent a necessity to do so?

Not all WAAS GPS approaches have a 250 ft lowest minimum. There are some that can come all the way down to 200 feet. This approach in Nevada, Missoura has 200ft ceiling and 1sm visibility.http://aeronav.faa.gov/d-tpp/1106/05831R2.PDF (If the link no longer works change 1106 to the current yymm)
I agree with Kent that for whatever reason: obstacle clearance, maximum permissable error on the GPS receiver, missed approach climb gradient and others affects how low the minimums are for that specific approach.

When the first LPV approaches were released, they were limited to a minimum DH of 250 feet. By the end of 2008, sufficient improvements were made to the WAAS system to allow for lower DH down to 200 feet. LPV approaches that have a DH 250 feet or higher, have a vertical integrity requirement of +/- 50 meters and those that go lower than 250 feet must meet a tighter vertical integrity of +/- 35 meters. In order to obtain the lowest minimums currently available for a LPV approach (200 DH and 1/2 mile visibility), several other criteria must be met by the runway. It must be at least 4200 feet long if I recall. It must have a parallel taxiway. The runway needs to have precision markings. There must be an approved approach light system. If these conditions are not met, then either the visibility or the DH or both are required to be higher. As of the beginning of this month, there were 470 LPV approaches published that have a DH of 200 feet. A complete list of the LPV approaches and the DH can be found at http://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/approaches/media/LPVs_060211.xls

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